1. Field of the Invention
The present invention relates to a production method of barium titanate powder used for forming a dielectric layer of a ceramic capacitor.
2. Description of the Related Art
As a dielectric layer of a ceramic capacitor, barium titanate (BaTiO3) is widely used. The dielectric layer is obtained by forming a green sheet from a paste containing barium titanate powder and sintering the green sheet. The barium titanate powder used in such an application is generally produced by a solid phase synthesis method. In the solid phase synthesis method, barium carbonate (BaCO3) powder and titanium oxide (TiO2) powder are wet mixed and dried; and then the powder mixture is fired at a temperature around 900 to 1200° C. to react barium carbonate particle and titanium oxide particle in a solid phase, so that the barium titanate powder is obtained.
Firing of the powder mixture of barium carbonate powder and titanium oxide powder is performed at the above firing temperature by raising temperature around normal temperature thereto. When firing of the powder mixture of barium carbonate particle and titanium oxide particle, generation of barium titanate starts approximately at 550° C. On the other hand, barium carbonate which is a raw material is known to show grain growth around 400 to 800° C. Therefore, the temperature rising process of the powder mixture results in grain growth of barium carbonate particle. Reaction at a predetermined firing temperature thereafter consequently causes to produce barium titanate powder with large particle size since barium carbonate particle of increased particle size is reacted with titanium oxide particle. In addition, the powder mixture used in the solid phase method is not always homogeneous in dispersion of barium carbonate particle and titanium oxide particle. Concentrations of barium carbonate particle are therefore varied in the powder mixture. In a portion with high concentration of barium carbonate particle, grain growth of barium carbonate particle is increased to produce large barium carbonate particle, but in a portion with low concentration of barium carbonate particle, grain growth of barium carbonate particle hardly occurs. As a result, particle sizes of barium carbonate particles are nonuniform, and particle sizes of the obtained barium titanate powder are varied. Further, a part of heat energy in the temperature rising process is used for grain growth of barium carbonate particle, resulting in a problem on energy efficiency.
Recently, it is required to downsize a capacitor, but a paste including barium titanate powder with large particle size prevents making dielectric layer thinner. Therefore, for making a dielectric layer thinner, the above-obtained barium titanate powder is subject to pulverization to produce powder having a desired particle size. However, pulverization is a time-consuming, expensive process, which makes powder properties of the obtained powder inhomogeneous. Further, when producing a capacitor by using barium titanate powder having greatly-varied particle size and nonuniform powder properties, electrical characteristic of the capacitor will be unstable. Therefore, a simple method to obtain homogeneous barium titanate powder having small particle size is required.
By suppressing grain growth of barium carbonate particle in the temperature rising process of the powder mixture, it may be possible to make the resulting barium titanate powder finer and to uniformize particle size thereof. The Japanese Unexamined Patent Publication 10-338524A discloses a production method of barium titanate powder to prepare powder mixture by mixing barium carbonate powder with relatively large particle size and titanium oxide powder with small particle size followed by firing process for suppressing grain growth of barium carbonate particle. Specifically, barium carbonate powder with specific surface area of 10 m2/g or less and titanium oxide powder with specific surface area of 15 m2/g or more are used. By this method, barium carbonate particle with large particle size is surrounded by titanium oxide particle with small particle size, so that the barium carbonate particles can be inhibited contacting each other, resulting in suppressing grain growth of the barium carbonate powder.
However, since the barium carbonate powder with relatively large particle size is used as a material powder, miniaturization of the barium titanate powder is limited. Also, in case of using powder with large particle size, reaction proceeds slowly, so that firing for long time or at high temperature is required for obtaining homogeneous barium titanate, causing a problem in energy efficiency.
Therefore, if it is possible to suppress grain growth of barium carbonate particle when using fine material powder, homogeneous barium titanate powder with small particle size may be obtained with excellent energy efficiency.
The Japanese Unexamined Patent Publication 2005-8471A discloses setting a temperature rising rate up to a firing temperature at 30° C./min or more for suppressing generation of hetero-phase in production of Ca-modified barium titanate ((Ba1-xCax)TiO3) powder. The temperature rising rate is specifically 30° C./min or 60° C./min in the Japanese Unexamined Patent Publication 2005-8471A.